In recent years, research and development are conducted on a nonvolatile memory device having memory cells that use variable resistance elements. A variable resistance element is an element that has a property that a resistance value changes (reversibly changes between a high resistance state and a low resistance state) according to an electrical signal and enables information to be written by this change in resistance value.
One structure of memory cells using variable resistance elements that is suitable for high integration is a cross-point structure. In the memory cells of the cross-point structure, each memory cell is placed at a different one of cross-points of orthogonally arranged bit lines and word lines so as to be provided between a bit line and a word line. Various types of such cross-point variable resistance nonvolatile memory devices are developed in recent years.
For example, Patent Literature 1 discloses a cross-point nonvolatile memory device using a variable resistor as a memory cell.
FIG. 21 is a block diagram showing a voltage system used in the nonvolatile memory device in Patent Literature 1. The voltage system 400 is a system which adjusts a voltage applied to a selected memory cell in the memory cell array at the time of read, and includes a sensing circuit 404, a bias circuit 408, and a differential amplifier 450. The sensing circuit 404 includes a first resistor 410, a sensing device 420, a selection diode 430, and a variable resistance element 440. A read operation can be performed by providing a read operation voltage to the selected memory cell comprises the selection diode 430 and the variable resistance element 440.
When an ambient temperature changes during the read operation and/or a process of the selection diode 430 has been changed, a threshold voltage of the selection diode 430 can be changed. For example, when the ambient temperature is increased, a threshold voltage can be decreased. When the threshold voltage of the selection diode 430 is changed in such a manner, a read operation voltage for the variable resistance element 440 can be changed.
In order to compensate a change in the read operation voltage, the bias circuit 408 holds a constant read operation voltage by generating a reference voltage to be applied to the variable resistance element 440. The bias circuit 408 can include a second resistor 460, a reference diode 470 having the same type as that of the selection diode 430, and a reference resistance element 480. The reference diode 470 provides a capability of tracking any change in the variable resistance element 440. When a reference voltage is generated in the bias circuit, the reference voltage is provided to the sensing circuit via the differential amplifier 450.
Patent Literature 1 discloses that a constant read operation voltage is held in the variable resistance element 440, regardless of any change in threshold voltage of the selection diode 430 by the application of a reference voltage generated by the bias circuit 408. Moreover, Patent Literature 1 discloses that a read operation voltage tracking a change in threshold of the selection diode is held and it is possible to apply the best voltage for read tracking a change in threshold of the selection diode.
Moreover, for example, Patent Literature 2 discloses a power source device using a differential amplifier.
FIG. 22 is a block diagram showing a power source device disclosed in Patent Literature 2.
The power source device 500 includes: an input terminal which inputs AC or DC power; a main circuit 511 which is connected to the input terminal and converts the input power into a DC output; a voltage drop element 513 which is connected to between one of the ends of the output of the main circuit 511, leads the output of the main circuit 511 to a DC output terminal, and restricts the back current; a first circuit (operational amplifier) 510 which compares a reference voltage 508 with a voltage obtained from one end of the output of the main circuit 511, and controls the output of the main circuit 511 by feeding the result back to the main circuit 511; a second circuit 503 which generates a signal in proportion to an output signal of the main circuit 511; and a third circuit 509 which generates, from the output signal of the second circuit 503, voltage drop characteristics having the effectively same characteristics as that of voltage drop characteristics of the voltage drop element 513, and adds it to the reference voltage of the first circuit 510.
In the power source device 500, a differential amplifier (called an operational amplifier in Patent Literature 2) is used, as the first circuit 510, for steadily stabilizing DC output voltage, and a feedback connection to the main circuit 511 is comprised.